This invention relates generally to fiber-optic, evanescent-wave biosensors and, in particular, to such a sensor that takes advantage of information derived from polarization orientation.
Optical fibers are being used in a variety of sensor applications. For example, as discussed in U.S. Pat. No. 5,494,798, a pair of optical fibers may be pulled into a fused biconical coupler and used without cladding to exploit the evanescent field present immediately outside the fiber coupler waist/air interface. If an antibody is attached to the exposed surface of the bare fiber, the evanescent field envelopes the molecule.
When an antigen subsequently attaches to the antibody, there are changes in the evanescent field can force a shift in the output coupling ratio. This results in an optically detectable characteristic signal.
Whereas previous fiber-optic evanescent-wave sensors utilized multi-mode fibers, the ""798 patent improved on the technique by employing a pair of single-mode optical fibers in a coupler arrangement. Light is introduced into one of the fibers to produce an evanescent region surrounding the coupling area, and the magnitude of light emitted from the pair of fibers is compared for detection purposes.
FIG. 1, taken from the ""798 patent, shows the overall fiber optic system generally at 10. Light from laser diode 14 is inserted into a first leg 17 of a fiber optic coupler 18, and exits on the same fiber at 19 (input channel). A second fiber 20 provides an output channel for light from the first leg 17. A first photo diode detector 21 is connected to the input channel and a second photo diode detector 22 is connected to the output channel.
Each detector feeds its own transimpedance amplifier. The outputs of the transimpedance amplifiers 23, 24 are applied to A/D converters 25 and 26 which provide digital electrical signals along wires 27 and 28 to an instrumentation board 29. The instrumentation board 29 is then connected to a personal computer 30 which provides outputs to a printer or a monitor.
The finished probe includes the coupler and attached antibodies, which yields a baseline ratio for the sensor. The finished probe is then exposed to a material of interest, and the ratio of the light through the two sides of the coupler changes as a function of the way in which the target attaches. That is, the localized index of refraction at the coupling region and the determination of the ratio is a function of the binding in the coupler region.
FIG. 2 is a graph which shows how index of refraction changes when the coupling region is immersed in solution and the antibodies attach. Note that the system is most efficient when the baseline ratio is on the steep portion of a curve as opposed to a local maximum or minimum. That is, it is best to operate at point xe2x80x98Xxe2x80x99 as opposed to, say, point xe2x80x98Y.xe2x80x99 By operating on the steep initial slope of the curve, very few antigens will cause a significant shift in ratio which is more easily detected.
In terms of the coupler itself, existing designs use off-the-shelf components intended for multiplexers and demultiplexers in telecommunications applications. Corning, for instance, makes these couplers by twisting together two or more 1300-nm, single-mode type SMF 9-125 optical fibers, heating up the twisted area and pulling the ends apart to create a necked-down (waist), nearly fused region. The number of fibers and other factors such as the proportion of each fiber in the twisted region determines the coupling ratio.
This invention improves upon the art of coupled fiber-optic, evanescent-wave biosensors through the use of configurations which detect changes in polarization for enhanced sensitivity. In contrast to existing techniques, wherein the fibers are twisted while pulled to disrupt polarization orientation, the inventive approach forms the necked-down region by heating and pulling the fibers without twisting them. As such, when polarized light is introduced, including randomly polarized light, the outputs will exhibit a split based upon polarization orientation as well.
One or more optical devices are then used to detect this change in polarization. In the preferred embodiment, polarizing beam splitters are employed to detect the split based upon polarization orientation. For example, light having more P-polarized light may emerge through one fiber, whereas more S-polarized light may emerge from another. Although the split in polarization may not be 100%, a system according to the invention may be appropriately modified to adjust the ratio of P to S levels.
One or more bindings partners are then attached to the necked-down region and within the evanescent field for very specific and direct detection of minute concentrations of an analyte of interest. The invention is applicable to any type of organic/inorganic material, so long as the interaction of one component causes a change in any optical property detectable by the apparatus. Interactions to which the invention is applicable include, but are not limited to, antigen-antibody, carbohydrate-lectin, receptor-ligand, binding protein-toxin, substrate-enzyme, effector-enzyme, inhibitor-enzyme, complimentary nucleic acid strands, binding protein-vitamin, binding protein-nucleic acid, reactive dye-protein, and reactive dye-nucleic acid interactions. The biomolecule may be linked to the surface of the fusion joint by means of a spacer molecule. Although the invention assumes the use of glass fibers, polymeric fibers may also be used in certain situations.